Boeing will deliver the first 767 tanker to the Italian air force for acceptance testing in mid-2007--eight months later than scheduled. The delay is the result of an unexpected vibration--at high refueling speeds--from the pylons connecting the aircraft's two probe-and-drogue refueling pods to the wings.

The problem was discovered last summer during flutter testing. "What we saw on the inboard side of the pylon was a flow separation," says Jeff Keller, Boeing's Italy 767 tanker program manager. "It has to do with the proximity of the pod and the pylon to the wing where the pressure peaks at high velocities."

The pylon's curved shape produced higher pressures and speeds (above Mach 1.0) on the inside of the pylon as the aircraft reached speeds above Mach 0.8. Boeing won't give an exact speed for onset of the "shock-induced, flow separation" problem, but some sources say the vibration became noticeable at Mach 0.84. The solution was to change from a pylon with curved sides to a straighter, constant-area, slab-sided design that offered no opportunity for the airflow to accelerate, separate and trigger the vibration.

"It was not a safety-of-flight issue," Keller says. He described the vibration as "minor, but different from what other 767 pilots had experienced." Analysis of the vibration's impact on the aircraft's fatigue life is not yet complete. "We have some more [analyses to do] once we start pod testing again later this summer."

Nonetheless, "we've already incorporated a new production design. It's complete and the pylon is being built as we speak."

Some critics had pointed to the Smiths wing-mounted aerial refueling pod design as the problem, but Boeing officials have attributed the vibration to their own pylon design and integration. Even though the problem was quickly identified and a solution found, the delay has rippled through the overall flow of modifications and delayed testing on the refueling pod. They also point to the problem of having only one aircraft in the 767 tanker test program at Wichita, Kan.

Boeing officials point out that they have to repeat all the testing for each of the three 767 configurations: 200 seats, a combination of 100 seats and cargo and all-cargo. Stacked up for the tanker program is interior testing for smoke detection and penetration, basic stability and control, flight controls and refueling testing for three hose and drogue positions (which starts this summer but extends into 2007) and the boom (this summer). The 767 tanker aircraft have so far accumulated more than 240 test hr. in more than 60 flights, they say.

Part of the delay will be used for "scope growth" of the IAF's tanker capabilities through customer-directed enhancements to the aircraft's interiors and avionics, Keller says.

Under the adjusted schedule, the first two aircraft will be delivered to the IAF in 2007 and the second two in 2008. The first tanker is now in Wichita, where it is engaged in the tanker test program. The second is finishing modifications in the Aeronavali facility in Naples, and it will be flown to Wichita in early 2007. As a second test asset, it is expected to speed up the program and to complete FAA certification testing. The third tanker is in Naples where it has started modification, and the fourth IAF 767 is to fly to Naples in 2007. All four are to be delivered to the IAF by early 2008. The 767 tankers built for Italy, Japan and the U.S. are to have a full self-defense subsystem.

The IAF had eyed the possibility of buying two more tankers, but defense budget reductions have put the idea on hold. Also, Aeronavali was to have provided modifications to foreign customers of an international 767 tanker program. The company is working on the Japanese tankers, but if Boeing doesn't win a U.S. tanker contract, it will have to fulfill its total offset obligations by offering work on other programs.

The IAF's delay has no impact on the delivery of its first 767 tanker to the Japanese Air Force, which is now slated for the end of the year, say Boeing officials. It is equipped only with the refueling boom.

The delay has actually affected the retirement of the IAF's four 707 tankers. The IAF was trying to save money by retiring them. Because of the delay, Boeing has agreed to pay the extra money to extend a support contract with Alenia and to bring the 707s back into service until the 767s are delivered so that IAF training doesn't suffer.

The older tankers are to revert to Boeing's ownership.

Company officials are close-mouthed about plans for the four aircraft, but at least one is destined for Omega, a service supplier company that offers refueling by the hour to the U.S. Navy for training and test purposes. The IAF 707 tanker has no boom, but its probe and drogue system is perfect for Navy and Marine Corps and Special Ops Command needs. Boeing will say only that there are plans to capitalize on the value of the aircraft.